I just performed the worst "upgrade" of all time on my laptop recently. I "upgraded" my Kubuntu 8.10 to 9.04, after having had good results with the same on my desktop.
Now my laptop is unusable; or at least not usable when you are actually using it directly. If you log in locally, you find XOrg using up to 98% of the CPU at all times. Even a konsole window is barely functional; don't bother trying to start an actual X application.
Oddly enough, this problem does not present itself when logging in over ssh; though that isn't a viable answer for a laptop.
I have seen that quite a few other people have found the same problem with Ubuntu 9.04, though I have yet to find a solution. Some people have been able to correct it by reconfiguring xorg, that did not work for me. Laptop in question has 2gbs ram, and a 1.6ghz P4m.

Which video driver are you using? Does switching to a different video drive help (i.e. vga or vesa)?

According to /etc/X11/xorg.conf I am not using a video driver at all, as none is specified there. I presume the location of the file that is pertinent to video drivers is somewhere else, but I don't know where.
I tried installing the other radeon driver, and it made things an order of magnitude worse (system no longer boots at all).

Xorg autodetects most items these days (not always correctly), so you need to check /var/log/Xorg.0.log to see what driver is being used. The log file might also point out what's causing such high CPU usage. Please attach it to a post here.

As Carpetsmoker suggested, try using VESA, by adding Driver "VESA" (in Section "Device") to xorg.conf.
If there's already a Driver entry there, comment it out along with "VendorName", "BoardName", etc.

Indeed the video driver does seem to be the key to my misery. Of course the mystery remains as to why it changed between version 8.10 and 9.04 of the same OS. We'll see if I can successfully reinstall it and get things back to some sense of normalcy, the VESA driver only works at 640x480 which is pretty well useless for what I need to do.

I think the problem may even more specifically come from the package "xorg-driver-fglrx", which is installed automatically when I try to install the radeon driver. I suspect this because if the radeon driver is present, but not used in xorg.conf (using driver "VESA" there instead), my system refuses to boot.

If I uninstall "xorg-driver-fglrx", using driver "radeon" in xorg.conf, my system boots but I get the runaway XOrg process (> 95% CPU as seen in top).

Similarly if the package "xorg-driver-fglrx" is installed and I use the radeon driver in xorg.conf, my system does not boot completely.

The only way I can currently get my system to boot and be semi-usable is to uninstall all the video drivers, and use the vesa driver. This is of course not even close to sub-optimal, as it loads at 640x480.

I have attached the Xorg.0.log and dmesg outputs from each of three conditions; running the vesa driver (radeon uninstalled completely), running the vesa driver (radeon installed but not used in xorg.conf), running the radeon driver.

I welcome any insight anyone may have on this. Bonus points for anyone who can explain why on earth kubuntu also decided to trash my wife's login account on my laptop.

I suspect that there is something very screwed up with your installation. Your GPU has not been supported by fglrx in over five years. The fact that Ubuntu insists on installing fglrx is a pretty big indicator that something is not right.

I could be wrong and the the high CPU usage could be related to the fact that you don't have direct rendering working with the radeon driver. What's the output of 'sudo modprobe radeon' ?

Ubuntu 9.04 is generally really broken if it comes to graphics support, for example, I was able to play Heroes of Might and Magic using WINE on Ubuntu 8.10 with native performance, but after a clean install of 9.04 on the same box, I lost all performance, and WINE strugled to do almost 10FPS in new 9.04 (and CPU usage was as usual, no CPU hog in this case), just broken drivers ... in RELEASE.

The only sollution is to ... go back to 8.10, updates that wanted to apply after clean install of 9.04 do not solve these problems.

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I will say that the graphics support has always been a dramatic disappointment on this system for me in kubuntu. I installed it on here a while ago, and much to my embarrassment my running streak of non-windows-driven presentations came to a screeching halt when kubuntu refused to work with the external monitor on this system in any way, shape, or form. It knew it was there (this is a laptop after all) but it would actually crash when I hooked it up.
I could hardly care less about graphics performance for games, or really graphics performance in general; I just need the correct resolution for image and presentation work. But when a system screeches to a halt in a console window because of previously-working drivers, that is unacceptable.
I'm going to wipe the kubuntu install, and go to FreeBSD 7.2. I previously has FBSD 6.1 on here, and life was good (except for the flash support, I'll have to put some time into that to make my wife happy).

Ubuntu 9.04 is generally really broken if it comes to graphics support, for example, I was able to play Heroes of Might and Magic using WINE on Ubuntu 8.10 with native performance, but after a clean install of 9.04 on the same box, I lost all performance, and WINE strugled to do almost 10FPS in new 9.04 (and CPU usage was as usual, no CPU hog in this case), just broken drivers ... in RELEASE.

The only sollution is to ... go back to 8.10, updates that wanted to apply after clean install of 9.04 do not solve these problems.

That's why all my wine-based gaming is done through my always faithfull FreeBSD 7.0 x86, I get good framerates and I know it won't break on the next updates. For the rest of my daily work I rely on M$-Linux 11.2m3 gnome and it's not done any weird thing so far.